[phicoh]

Could be a UI thing. When the screen is black you expect the monitor to be off and press the on-off switch.

Same thing with switching monitors off after work.

Though same screensavers would run for a while and then switch the monitor to power saving mode, effectively turning it off.

[Symbiote]

CRTs were in use for decades.

In the early 1990s, there was no power saving. You could turn off the monitor with the front power switch, or use the screensaver.

A few years later, power saving options began to be introduced. The computer could signal to the monitor that it should go into a low(er) power state. Turning on again took at least 5 seconds for a visible picture, sometimes 15-20. That might not be appropriate if the computer had to be used in response to something like a phone call, or visitor at reception.